Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.
aid 1, aide (nn.)
Both mean “helper or assistant,” but aide is a clipped form of aide-de-camp, the military title, and is always the spelling for military contexts. Americans use both spellings in titles for assistants, although aide may be slightly more frequent. Visual aids, hearing aids, and other helpful objects are always spelled aid.
